The importance of monitoring individual steps performed during the semiconductor manufacturing process is well established. To date, the two options that have been available for monitoring these processes have been ex situ metrology; process monitoring performed after the wafer has been processed, or in situ sensing, monitoring the process while the wafer is being processed.
Although adequate results can be achieved by ex situ metrology alone, the limitations of this latter approach are twofold. First, ex situ metrology is often a throughput-limiting step. As a result, a sampling schedule is used, and therefore not every processed wafer is measured. This limits the ability to detect manufacturing flaws, and also allows flawed product to move to subsequent processing steps. A worse case scenario is that any flaw can only be detected in final test, therefore resulting in maximum waste. Second, some ex situ metrology measurements, such as, for example, electrical test, require additional process steps before the test can be performed. Inherent to these measurements is the risk of performing additional work on a flawed part.
Although not as accurate, in situ sensing offers the advantage of a fast measurement that provides information about the process step of interest, or a very recent process step. In situ sensing can almost always be performed on every processed wafer with no loss of throughput. In addition, in situ sensing can diagnose the step it is sensing without the requirement for subsequent steps to be performed. A limitation of in situ sensing, however, is that it is not a direct measurement of wafer outcome. Rather, process conditions are sensed, and the impact on wafer outcome and device performance is inferred via physical, phenomenological, or empirical modeling.
Numerous analytical instruments exist to sense a wide range of compounds that are relevant to microelectronics manufacturing. However, far fewer sensors capable of withstanding the processing environment are available.